1. Field of the Invention
The present invention relates to fiber optic gyroscopes, and in particular to the use of depolarized light in fiber optic gyroscopes without input polarizers, and to the use of ordinary non-polarization preserving fiber in the gyro coil.
2. Discussion of the Related Art
Two events in the development of the fiber optic gyroscope that have been important in obtaining theoretical sensitivity levels and good bias stability have been the recognition of the "minimum configuration" with its unique single mode operation, and the use of polarization preserving fiber to virtually eliminate polarization noise. See S. Ezekiel and H. J. Arditty, in Fiber-optic Rotation Sensors. Ed., S. Ezekiel and H. J. Arditty, Springer-Verlag, New York p. 2-26, 1982; and W. K. Burns, R. P. Moeller, C. A. Villarruel and M. Abebe, Opt. Lett. 8, 540 (1983). Nevertheless interest continues to be present to reduce instrument cost for less demanding applications through the use of configurations with reduced component complexity and cost. Today this particularly applies to the use of polarization preserving high birefringence fiber, the cost of which remains high. Alternative configurations include the 3.times.3 coupler, which passively provides operation at its maximum sensitivity (quadrature) point; and the use of depolarized light in fiber gyros, which was shown theoretically to be equally effective (ideally) in controlling birefringence induced bias drift as the use of polarized light. See S. K. Sheem, Appl. Phys. Lett. 37, 869 (1980); G. A. Pavlath and H. J. Shaw, Applied Optics 21, 1752 (1982); and W. K. Burns, C.-L Chen and R. P. Moeller, J. Lightwave Tech. LT-1, 98 (1983).
The use of depolarized light in fiber optic gyroscopes was first considered by Pavlath and Shaw, cited above, who reported in the ideal case (degree of polarization P=0) that fiber birefringence effects caused a scale factor variation, but that no phase bias was added to the Sagnac phase shift. Similar modeling, which more explicitly included the effects of broad-band sources, appeared in Burns, Chen and Moeller, cited above. The fiber gyro with a 3.times.3 coupler input led naturally to depolarized operation because the superluminescent diode sources were already only partially polarized, and because a single polarizer could not be used in the physically different input and output paths in the 3.times.3 coupler configuration. This argument led to the demonstration of reduced polarization noise with reduced degree of polarization in a 3.times.3 coupler configuration. See W. K. Burns, R. P. Moeller and C. A. Villarruel, Electron. Lett. 18, 648 (1982). A recent variation of this configuration has appeared, where a signal processing scheme is proposed to cope with the birefringence induced scale factor variation. See H. Poisel, G. F. Trommer, W. Buhler, E. Hartl, and R. Muller, Electron. Lett. 26, 69 (1990).
U.S. Pat. No. 4,997,282 to Pavlath generally describes technology components for a depolarized fiber optic gyroscope; however, the gyroscope requires polarizers between a gyro coil and a coupler. U.S. Pat. No. 4,529,312 to Pavlath et al. describes a fiber optic gyroscope including two 2.times.2 couplers, a gyro coil and a polarizer disposed between one coupler and the gyro coil. U.S. Pat. No. 5,046,848 to Udd describes a fiber optic gyroscope including two 2.times.2 couplers with a polarizer disposed therebetween. U.S. Pat. No. 4,869,592 to Bergh describes an apparatus for obtaining a measurement and absolute rotation from a closed loop fiber optic gyroscope, the gyroscope having two 2.times.2 couplers with a polarizer disposed therebetween, and a gyro coil; however, the gyroscope does not disclose a depolarizer disposed between the gyro coil and a coupler. German Patent Publication DE 3115-804 to Bohm et al. described a fiber optic gyroscope having two 2.times.2 couplers with a polarizer and a depolarizer. U.S. Pat. No. 4,653,917 to Moeller et al. describes a fiber optic gyroscope having a 3.times.3 coupler, a polarized light source and a gyro coil; however, the gyroscope does not contain a depolarizer connected to the gyro coil. U.S. Pat. No. 5,080,488 to Buehler et al. describes a method for evaluating signals from a fiber optic gyroscope, the gyroscope including a 3.times.3 coupler and a gyro coil; however, the gyroscope does not contain a depolarizer connected to the gyro coil. U.S. Pat. No. 4,556,293 to Burns et al. describes a broad band unpolarized light source.